It's 50 feet from the centre of a carriageway, not a public right of way (e.g footpaths aren't covered by the legislation). You can shoot closer than that to a carriageway, provided a user isn't “injured, interrupted or endangered”.
Injured and endangered are covered by having an adequate backstop. “Interrupted” is slightly ambiguous. Since the legislation dates back to horses, it could be argued “interrupted” means telling them to wait or take a detour, rather than them stopping to rubberneck.